1. Field of the Invention
The present invention relates to a fetal electrode and, in particular, to a fetal spiral electrode which includes a drive rod, a clutch, a handle, and a safety clip integrally molded together to form a channeled drive mechanism for imparting the torque required to attach the electrode to the fetus.
2. Description of the Related Art
It is desirable to monitor fetal heart rate continuously during labor and delivery in order to detect fetal distress. Devices which are external to the mother's body do not adequately isolate the fetal and maternal heartbeats. Consequently, devices which attach directly to the fetus during labor are now commonly used. U.S. Pat. No. Re. 28,990, issued to Hon et al., discloses a fetal spiral electrode (FSE) assembly commonly used to monitor fetal heart rate during birth.
The conventional fetal spiral electrode assembly includes a curved guide tube of adjustable shape for insertion of the fetal spiral electrode through the mother's cervix and into contact with the fetus during labor. A plastic tip or holder is slidably received in the guide tube. A sharp, pointed, fetal spiral electrode is mounted on the forward end of the holder for contacting the fetal epidermis.
A reference (maternal) electrode in the form of a flat fin or plate is electrically isolated from the fetal electrode and located on the rear end of the holder. A flexible, hollow drive tube with a cutout on its forward end fits inside the guide tube and engages the holder. The cutout of the drive tube engages the reference electrode in the holder to impart translation and rotation to the holder and, hence, to the fetal spiral electrode. A handle on the opposite end of the drive tube allows the user to push, pull, and rotate the drive tube within the guide tube. A forward-twisting force is applied to the drive tube to affix the fetal spiral electrode in the fetal epidermis.
The two electrodes are connected to separate wires, which are threaded through the common center of the drive and guide tubes until they ultimately exit at the proximal end of the drive tube. After the fetal spiral electrode is secured to the fetal epidermis, the drive tube and guide tube are removed by pulling the tubes longitudinally over the wires and away from the mother. Removal of the drive and guide tubes leaves the electrodes, the holder, and the wires in place inside the mother. The uninsulated ends of the wires opposite the electrodes are then connected to a fetal monitor.
Manual connection of the uninsulated ends of the wires is cumbersome and risks shorting the wires. If shorted, the wires cannot transmit correct signals from the fetal and reference electrodes. Accordingly, a connector can be added to the fetal spiral electrode assembly disclosed in the '990 patent. As taught by U.S. Pat. Nos. 5,205,288 (issued to Quedens et al.), No. 5,199,432 (issued to Quedens et al.), and No. 5,168,876 (issued to Quedens et al.), the connector solves the problem of manual connection of the uninsulated ends of the wires. Because the guide and drive tubes are removed by pulling them longitudinally over the wires and connector, however, the connector must have an outer dimension which is smaller than the inside diameter of the drive tube (and, of course, the larger-diameter guide tube as well).
The wire connected to the fetal spiral electrode and the wire connected to the reference electrode form a twisted wire strand which enters the connector through a strain relief element. The wire from the fetal spiral electrode is connected to a first, gold, terminal or ring contact; the wire from the reference electrode is connected to a second, gold, terminal or ring contact. The terminals are electrically and physically separated by a spacer. The connector has a forward tapered tip.
The connector engages a support plate, which is affixed to the expectant mother (typically to the thigh) and provided to support the connector. Upon insertion of the connector into an opening of the support plate, the two ring contact terminals on the connector click into physical and electrical contact with two corresponding barrel contacts in the support plate. Moreover, the tip of the connector abuts a wall in the support plate to prevent over-insertion of the connector.
The support plate carries its own ground electrode. Consequently, three electrical circuit paths are created upon engagement of the connector with the support plate: (a) fetal electrode to a first wire to a first contact terminal to a first barrel contact to a first output terminal to the monitor; (b) reference electrode to a second wire to a second contact terminal to a second barrel contact to a second output terminal to the monitor; and (c) ground electrode to a third output terminal to the monitor.
To use the fetal spiral electrode product having a connector, the shape of the guide tube is adjusted and the guide tube is inserted through the mother's cervix and into contact with the fetus. Care must be exercised to assure that the sharp fetal spiral electrode does not extend out of the guide tube during insertion; otherwise, risk to the patient of injury and infection arises. Once the guide tube contacts the fetus (and is held against the fetus using one of the user's hands), the drive tube is advanced (using the second hand) until the fetal spiral electrode contacts the fetus.
While pressure is maintained against the fetus by the guide tube and drive tube, the drive tube is rotated, using the second hand and the handle, until the fetal spiral electrode is secured to the fetal epidermis. Typically, one full revolution suffices to secure the fetal spiral electrode. Then the drive tube and guide tube are removed, leaving the electrodes, the holder, and the wires in place inside the mother, by sliding them over the electrode wires and connector. Finally, the connector is plugged into the support plate.
The connector must be fully inserted into the support plate to assure optimal signal quality. The connector of the conventional device has a constant diameter along its length. The device cannot provide any visual assurance, therefore, that the connector has been fully inserted. This is one drawback of the conventional device.
A second drawback associated with the conventional fetal spiral electrode assembly described above is the potential for the fetal spiral electrode to extend out of the guide tube, during storage or transportation, before the fetal spiral electrode assembly is ready for use. If exposed, the sharp fetal spiral electrode can pierce the package, typically a paper and plastic pouch, in which the assembly is stored and transported. A person handling the electrode (or the patient) may then be harmed and sterilization of the electrode is jeopardized. In addition, the electrode itself may be damaged.
A related problem associated with the fetal spiral electrode assembly described above is the potential for the fetal spiral electrode to extend out of the guide tube during the initial stages of use. Such premature extension may injure the patient and may cause infection. The problem of premature extension of the fetal spiral electrode out of the guide tube, before or during the initial stages of use, has been solved by the packaging system disclosed in co-pending U.S. patent application Ser. No. 08/126,222, filed on Sep. 23, 1993, entitled "Packaging System for a Fetal Electrode" and incorporated herein by reference.
Another problem associated with the conventional fetal spiral electrode assembly is that the wires and the connector, which convey the electrical signal from the fetus to the monitor, must traverse laterally through the hollow center of the drive tube. This means that the connector necessarily must have an outer dimension smaller than the internal diameter of the guide and drive tubes. Because the guide and drive tubes must be small in diameter in order to transit the closed cervix, this, in turn, means that the connector diameter must be relatively small.
The requirement of a small-diameter connector has several disadvantages. First, the clinician must grasp and handle the connector to insert it into the corresponding socket of the support plate. The smaller the connector, the more difficult it is to handle. Second, a proper connection of the connector to the support plate must be ensured. A smaller connector of constant cross-section is unable to provide assurance that the required connection has been achieved. Finally, the support plate and fetal spiral electrode operate in a fluid-filled environment. A smaller connector risks an inadequate seal of the opening in the support plate into which the connector is inserted. Absent an adequate seal, fluid from the environment may enter the opening in the support plate and adversely affect the connector-socket electrical connection or the other electrical circuit paths discussed above.
U.S. Pat. No. 4,644,957 recognizes the drawback, that the connector must necessarily be of a diameter smaller than the guide and drive tubes, characteristic of the fetal spiral electrode assembly described in the '990 patent. The '957 patent solves that problem by placing the wires alongside a solid drive wrench (rather than inside an annular drive tube) and by providing a slotted guide channel with a C-shaped cross-section (as opposed to a solid, annular guide tube). The wires reside freely inside the guide channel and parallel to the drive wrench. Because the wires are of a smaller diameter than the width of the longitudinal slot in the guide channel (enabling the wires to exit the slot), they must either be wound in a spiral around the drive wrench or positioned in the guide channel away from the slot to retain them securely inside the guide channel. After the fetal spiral electrode is secured to the fetus, the drive wrench is pulled out of the drive channel. The guide channel is then withdrawn, in a similar manner, as the wires slip freely out of the longitudinal slot in the guide channel.
The solution presented by the '957 patent has its own difficulties. The wires must be sized so that they are smaller than the width of the longitudinal slot in the guide channel. Thus, the size of the wires is restricted and the wires may exit the slot prematurely. More importantly, the wires reside freely inside the guide channel and may affect rotation of the drive wrench. The wires may become entangled around the drive wrench, in the worst case, preventing both rotation and removal of the drive wrench. The risk of entanglement is especially great if the wires are purposefully wound in a spiral around the drive wrench. Finally, the wires may not be aligned with the slot, after the drive wrench is removed, rendering withdrawal of the guide channel difficult.
Still another problem associated with the fetal spiral electrode assembly described in the '990 patent is that the drive mechanism consists of the handle and a separate drive tube. The plastic, molded handle is pressure-fit onto the extruded drive tube. When the user rotates the drive handled it is assumed that the drive tube rotates by the same amount as the drive handle and that the holder and fetal spiral electrode, in turn, rotate commensurately. The pressure fit of the conventional two-piece drive mechanism may inadequately transmit rotational motion between the handle and the drive tube; some slippage may occur. If so, the drive tube rotation (and, therefore, the fetal spiral electrode rotation) will not directly reflect the handle rotation.
The two-piece drive mechanism also fails to provide the user with consistent and optimal tactile feedback. The user of the fetal spiral electrode assembly described above turns the drive handle until a mild resistance is felt. Such resistance indicates that the fetal spiral electrode has been securely attached to the fetus. Because slippage may occur between the handle and the drive tube through the pressure fit of the conventional two-piece drive mechanism, the user may not receive the tactile feedback desired. Consequently, the user lacks confidence that a secure attachment of the fetal spiral electrode has been achieved.
Still another problem associated with the conventional fetal spiral electrode assembly is that the electrode wires must be straightened completely before the guide and drive tubes are pulled over the wires and connector. Otherwise, the wires may drag, catch, or snag on the drive tube, as it is removed, placing tension on the fetal spiral electrode. Such tension may pull the fetal spiral electrode out of engagement with the fetus.
Graphic Controls Canada Limited sold a fetal ECG electrode product during the mid 1980's in Canada, under the MEDI-TRACE.RTM. trademark, which incorporated a slotted drive rod to eliminate routing of the wires through a hollow drive tube. The open slot in the drive rod provided for a clean release of the electrode wires after attachment of the fetal spiral electrode, thus reducing wire friction and tugging during removal of the drive rod.
The product was unsuccessful, in part, because the solid, slotted drive rod was inflexible (too rigid and stiff) in comparison to its hollow tube counterpart. The stiffness of a drive rod in a fetal spiral electrode assembly must balance competing requirements. First, the stiffness must be sufficient to (a) transmit torque directly from the handle to the holder and fetal spiral electrode, (b) provide the "feel" required to assure that the fetal spiral electrode is attached to the fetus without over-rotation which would risk damage to the fetal scalp, and (c) allow sufficient bend so that the fetal spiral electrode can be inserted comfortably into the mother.
The Canadian product was overly stiff (insufficiently flexible) and, accordingly, failed to provide the feel required to avoid over-rotation. Consequently, the product risked fetal injury: pulling a plug of skin out of the fetus upon over-rotation of the handle. The clinician was required, therefore, to exert the utmost care to avoid over-rotation.
The risk of injury presented by the Canadian product might have been reduced by a clutch mechanism at the interface between the drive rod and the holder. Such a mechanism would transmit a torque sufficient to affix the fetal spiral electrode to the fetus but slip, or disengage, if a larger torque were applied. The Canadian product did not have such a protective mechanism. In short, therefore, the Canadian product was unsuccessful because it was unable to achieve the same torque characteristics as its conventional, hollow, drive tube counterpart.